1. Field of the Invention
The present invention relates to a steering gear apparatus of the rack-and-pinion type adapted for use in a steering system of an automotive vehicle.
2. Description of the Prior Art
In a conventional steering gear apparatus of this kind, the toothed portion of the pinion is in the form of a helical gear, and the axis line of the pinion is inclined at a predetermined angle relative to the normal line of the rack member such that a steering effort is applied to the meshed portion of the toothed portions of the pinion and rack member at the predetermined angle relative to the axis line of the rack member. To eliminate looseness at the meshed portion, the rack member is resiliently supported by a rack guide which is carried by a spring member to bias the rack member toward the pinion.
In the conventional steering gear apparatus, as shown in FIG. 12, a tooth 1a (a helical toothed portion the tip circle radius of which is r2 and the pitch circle radius of which is r1) of a pinion 1 is disengaged from teeth 2a of a rack member 2 at its tip and root and is in sliding contact with the teeth 2a of rack member 2 at two flanks a and b in a tooth trace direction for effecting a wedge effect. In transmission of the steering effort at the meshed portion of the teeth 1a and 2a, there will occur a frictional force due to sliding at the tooth flanks a and b, and the rack member 2 is slightly rotated about its axis line or deviated from its axis line due to a counterforce acting against the frictional force when applied with a rotational force from the pinion 1. (see FIGS. 9(A), 10(A) and 11(A)) This results in deterioration of the steering performance. The frictional force Fp caused by sliding at the tooth flanks a and b is determined by a biasing force of the spring member (not shown) for biasing the rack member 2 toward the pinion 1, a normal line force F1 (a force acting in a direction perpendicular to the engaged tooth flanks) defined by an angle .theta. (usually, 29.degree.-40.degree.) between the tooth flanks a and b and a frictional coefficient .mu., as is represented by the following equation; EQU Fp=.mu..times.F1=(.mu..times.Fo/2)/sin (.theta./2);
Where Fo is a numerical representation of the biasing force of the spring member.
To improve such deterioration of the steering performance as described above, Japanese Utility Model Laid-open Publication No. 2-33171 discloses a steering gear apparatus wherein a rack member is provided with a rib portion for restriction of rotation, and wherein a housing containing the meshed portion of the rack member and a pinion is provided with a restriction means for engagement with the rib portion of the rack member to restrict rotation of the rack member in a condition where the rack member is slidable only in an axis line direction. In the steering gear apparatus, it is, however, required to provide a space for the rib portion and restriction means in the housing. As a result, the steering gear apparatus becomes large in size and heavy due to the provision of the rib portion and restriction means.